In conventional semiconductor packaging processes, a resin material is used to encapsulate the semiconductor die. Since these packages need to be connected to printed circuit boards or other devices, it is necessary to avoid depositing the resin on the electrical contact leads. The typical mold cavity does not form a perfect seal with the surfaces and edges of the device enclosed by the cavity. Wherever gaps exist between the mold cavity and the device, there is a potential for resin material to be deposited in unwanted areas. This unwanted layer of mold compound resin, or resin and fillers is called flash. The thickness of the flash can vary from a thickness of a few microns up to a thickness of tens of microns and depends on the composition of the mold compound (e.g. epoxy resin and filler distribution and size), the mold press clamping tonnage, the mold pressure used, and the design of the tool cavity. When flash forms on electrical contact leads, for instance, post-plating operations cannot proceed since metal cannot be deposited on the insulating layer. Typically, flash can be removed by a number of operations such as, but not limited to, sandblasting, wet chemical exposure, and flame-off.
Flash is especially troubling when implementing the solder uplink concept disclosed in U.S. Pat. No. 6,364,542, “Device and method for providing a true semiconductor die to external fiber optic cable connection”, which is hereby incorporated in its entirety by reference. FIG. 1 illustrates a cross-sectional view of a stacked molded package as constructed according to current manufacturing techniques. The solder uplink concept, illustrated in FIG. 1, involves the fabrication of a stacked molded package 100 by connecting a mother package 105 to a daughter package 155 through solder bump pads 115. The problem is that it has been found that the typical transfer molding operation with standard mold tooling creates a thin flash layer covering some or all of the top surfaces of the solder bump pads 115 that are intended to be exposed through the molding material 135 on the mother package 105. FIG. 2A illustrates an isometric view of the injection molding process. In FIG. 2A, the molding compound 135 is shown flowing across the mother integrated circuit die 110. Note that the inside bottom surface 210 of the mold cavity 215 makes contact with the top surface of the solder balls 115. Note further the formation of flash 200 on the top surface of the solder balls 115. Specifically, flash 200 is represented where molding material 135 flows onto the top surfaces of solder balls 115. This occurs because a standard mold chamber has surfaces that are not completely smooth. Typically, a mold chamber surface will have a roughness of approximately 1.2 RA (micron average roughness) or more. The roughness of the mold chamber surface allows gaps 205 to form between the inside surface 210 of the mold chamber and the top surface of the solder balls 115. Molding resin and/or fillers seep between inside bottom surface 210 of mold cavity 215 and the tops of the solder bump pads 115. FIG. 2B illustrates a magnified view of how molding material 135 flows into gaps 205 left between inside bottom surface 210 and the surface of solder bump pads 115, causing flash 200. In this case, flash is catastrophic since the resin layer prevents good mechanical and electrical contact between solder balls 115 on mother package 100 and solder balls 150 on daughter package 155.
Unfortunately, conventional means of removing flash are not desirable since they are slow and require additional equipment. It is desirable to develop techniques for creating flash free solder bump contacts on the top surface of semiconductor packages without the use of additional equipment or processing steps. Eliminating flash in the production of the mother package 100 will simplify the manufacturing steps, reduce costs, and enhance the reliability of the resulting module.